Fluidized bed combustors have been known and used in the past for the burning of coal and other fossil fuels. However, the combustors have been deficient for one or more reasons. Typically, conventional combustors require that coal particles must be crushed to a particular particle size before the coal can be burned. This requires additional time and equipment and represents an increase in the cost for the recovery the energy in coal.
Conventional fluidized bed combustors have also been inefficient in the removal of coal fines from the combustion gases generated in a primary combustion area of such a fluidized bed combustor. This has typically required reinjection of such fines back into the primary combustor area which, in turn, requires additional equipment and adds to the overall equipment and operational costs.
Sulfur removal from the combustion gases has also been a problem and expensive, elaborate scrubbing equipment has been needed to remove sulfides from the combustion gases, thus further adding to the overall cost of the equipment.
A fluidized bed combustor known by the name Ignifluid boiler has been developed by Albert Godel and marketed by Fives-Cail Babcock. A system of this type uses a moving grate for use as a distributor to burn coal in a fluidized bed, and the system uses substoichiometric air in the fluidized bed. However, the bed operates at a relatively high temperature, such as in the range of 2000.degree. F. to 2500.degree. F., has a relatively low turndown ratio (a measure of heat control from the combustor), a high NO.sub.x generation, and no substantial removal of sulfur from combustion gases. A disclosure of this system is found in an article entitled: "A New Combustion Technique" by Albert Godel, Engineering and Boilerhouse Review, May 1956. Another disclosure of this system is found in an article entitled: "Studies Toward Improved Techniques for Gasifying Coal" by R. A. Graff, City College of New York, Final Technical Report, NSF Grant GI34286A-1, July 31, 1976.
In view of the drawbacks in conventional systems, a need has arisen for an improved fluidized bed combustor which avoids the above-mentioned problems of conventional combustors.